Hum elimination means



Nov.. 18,` 1947. E. ToTH HUM ELIMINATION MEANS 'Filed Feb. 19, 1945 4- -T-ZO l fla EMERICK TOTH awww Patented Nov. 18, 1947 UNITED STATES PATENT OFFICE i t 2,430,894 v 1 HUM ELIMINATION MEANS Emerick Toth, Cheverly, Md. Application February 19, 1945, lSerial No. 578,759

(Granted under the act of March 3, 1883, as

3 Claims.

amended April 30,- 1928; 370 O. G. 757) This invention is directed to the problem of m extraneous voltages induced in circuits employing tubes with indirectly heated cathodes.

It is an object of the invention to eliminate such voltage.

It is a further objectl of the invention to eliminatey induced voltages while permitting signal level 'gain variation within the elimination network. Y

The invention will be described with reference to the exemplary embodiments shown in the drawings, in which: 1

Figure 1 shows an application of the invention toan interstage coupling circuit,

Figure 2 shows in circuit diagram certain components of Figure 1,

, Figure 3shows an application of the invention to a variable gain interstage coupling circuit, and

Figure 4 shows-in circuit diagram certain components of Fig. 3.

The circuit of Figure 1 comprises an interstage coupling circuit in which tube I is the driving tube andtube 2 is the driven tube. The driving tube is characterized in that its output is develcped across a cathode load. In the circuit of Figure 1,.'tube I is la Vpeak-limiting diode which is operative on the output of detector tube 3. Tubel 3 is a diode having a load resistance comprising resistors lI` and `5, and is energized by a modulated intermediate or radio frequency signal introduced by transformer 6.

Anode 'IA of the limiting diode is connected intermediately to the detectorload resistance be-` tween resistors 4 and 5. Its cathode 8 is connected through series resistor 9 to the junction of the condenser Il and series resistor I which are connected across the load resistance as shown.

Cathode 8 of diode I is provided with an indirect heater I2 which is normallyenergized by a low-frequency alternating current. In tubes embodying indirectly heated cathodes there is distributed capacity between the heater element and the cathode structure. In the usual application of such tubes, circuit means are employed Wherein the impedance between the cathode itself and ground is very low at the frequency at which the heater is energized, and consequently the above mentioned residual capacity does not cause the development of an appreciable voltage swing at the heater current frequencies. In circuits in which the cathode is at relatively high impedance to ground, the capacitative current will develop a substantial A. C. voltage across the cathode load which appears in the output of the stage as a hum voltage. The circuit shown in Figure 1 is representative of the type of operation. I-Iere heater I2 is energized through leads I3 and It, lead I4 being grounded, With this connection there is 'an effective capacity connected between cathode 8 and lead I3 of the heater circuit.

` The output of driving tube I'is coupled through condenser I5 and appears across resistor I6. As the output includes the desired signal component and hum components induced from heater I2, these voltages are applied to amplifying tube 2 between control element I'I and ground, but the potential of cathode I8 is controlled so as to eliminate the'undesired effects of the hum components added to the signal.

This is effected by providing `a series resistor I9 connected to cathode I8, and additionally coupling cathode I8 to the ungrounded heater lead I3 through condenser 20; Condenser 20 is selected ofsuitable magnitude so that in combination with the distributed capacity between cathode I8 and its heater 2|, cathode I 8 will have induced `thereon a voltage of substantially equal phase and magnitude with that of cathode 8 of tube I Consequently the hum voltage components which are coupled through to control element I'I of tube 2 do not cause a resulting shift in the grid-cathode potential of amplifying tube 2.

The operation of the circuit of Figure 1 will be more apparent from an inspection of Figure 2, in which certain circuit elements in which the hum and signal components are present are shown in bridge form. The hum voltage may be considered to be introduced at terminals 22 and 23 of Figure 2. Condenser 2| constitutes the distributed cathode-heater capacity of diode I. Inasmuch as the reactance of condenser II may be substantially neglected in View of the magnitude of resistance 9.andcondenser I5 has very low reactance with comparison to the resistance I6, a suitable value for condenser 20 (which forms one arm of the bridge) may be selected to establish zero voltage between control element I1 and cathode I8 of tube 2 with respect to the undesired hum voltage. In this connection it will be noted that condenser 20 as shown in Figure 2 may be considered to include the distributed capacity between cathode I8 of tube 2 and its heater 2l. The bridge arrangement formed by the inclusion of condenser 20 is in balance as regards the undesired hum voltage but out of balance as regards the desired signal voltage, so that the desired signal is unaffected by the bridge,

The circuit shown in Figure 3 is similar to that of Figure 1 but illustrates the employment of the invention in connection with a circuit where variation of the desired signal input level to control element I'I is required Without substantially affecting the hum voltage balance.

In order to obtain signal level variations in the circuit of Figure 3, voltage divider 25 is coupled to condenser I5 and is connected :also :to cathode I8 of ,tube .2. The signal delivered Irom'voltage divider 25 is coupled through condenser 26 to resistor I6 connected with control element IJ of tube 2. The grid-cathodesignal on tube 2 is determined by the adjustment of voltage divider 25.

'Ihrough the operation of condenser 20, however, the induced hum voltages are prevented from affecting the grid-cathode potential in tube 2. This may be better seen from the diagram shown in Figure 4. Here, .as Yin :Figur-e 2, the hum voltages may be considered to 'be 'supplied at terminals 22 and 23. Condenser 20 again is connected so that no voltage of heater supply frequency is .developed across the .bridge .diagonal consisting .of condenser I5 and voltagedwider 2:5. Consequently, inasmuch .as the valueof ,resistance I6 is much greater than :the impedance of con* denser .26, control element ,-Ifl of the tube .2 may be considered .to .be connected directly to `the voltage divider. No hum voltage components are developed yacross the grid-cathode irl-pnt -of tube 2 iat any setting :of the volume .control voltage divider 25 tprovided. itl-iat the value .of resistor I6 is large compared to resistor 48. 'This is normally the case and .the voltage oom.- ponent will be substantially lzmlldd :from fthe output circuit .of tube :2.

lI t will be understood that the embodiments shown and 'described are exemplary only .and are not to be considered as 'limiting the invention, the scope whereof is dened in fthe fapllended claims.

The invention :described herein may be man-u.- factured and used by or for the Government of the yUnited States of America for governmental purposes without the payment of `am' royalty thereon or therefor..

I claim:

l. An interstage transmission vcircuit comprising a driving tube having a cathode and a heater therefor, ,Said heater ,having a capacity Ain relation to said cathode and `loeing operative on energy-ization to effect a potential variation thereof, means for controlling the cathode poten.- tial in accordance -with a control signal, an impedance driven by the cathode, a Asecond tube having a control element and cathode, `an impedance in series with the second tube cathode, means connecting the control element withthe rst tube cathode, and .a ,capacitor connected between the second tube .cathode and Athe first tube heater operative to drive said second cathode in substantially the same potential phase and amplitude as the hum potential Variation of the first tube cathode.

2. An interstage transmission circuit comprising a driving tube having a cathode and a heater therefor, -said heater having a capacity in relation to said cathode and being Loperative on venergization to eiTect a hum potential variation thereof,

zmeans for controlling the cathode potential in .accordance with a control signal, a resistance vconnected to the cathode, a second tube having a control element and a, cathode, a resistance in series with the second tube cathode, a capacitor Iconnected between the second tube cathode and theirst tube heater operative to drive said second cathode in substantially the same potential phase :and Iamplitude fas the hum potential variation of the first tube cathode, a resistance connected between the first and second cathodes, and means for .adjustably coupling the control element to said :last mentioned resistance.

3. A limiting detecting and amplifying l.circuit including a diode having Ia .load resistance, means for introducing a signal in series with ,said load resistance, a ,condenser connected to the .positive end of the load resistance, a series resistor connecting the condenser to the .other end of the load resistance, a resistance connected to the junction of the `.condenser and .senies resistance, a second idiode :having :a cathode connected fto the last mentioned resistance, an anode .connected to an intermediate point on the .load resistance. and a heater for its cathode, said heater having a capacity in relation to saidloathode, an vamplifying tube including a cathode .and a .control elament, .a resistor connecting said iast mentioned Cathode 'with the ,positive ,end fof the 4load resistance, means .coupling ithe ycontrol .element Yto :the cathode of the second diode, and a condenser connected between the amplifying tube Acathode and the .Second diode heater operative to cause asimilar potential variation in sai-d .cathode as established in the second diode heater `on A. 1C. heater operation.

EMERCK TOTH.

CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name 'Dat-e 1,862,020 Krahl June '7, 1932 2,223,206 Dome Nov. 26, 1940 FOREIGN PATENTS Number Country Date 430,448 Great Britain June 119, 1935 

